In the production of edible oils and edible fats the oil is refined and neutralized and bleached oil is then supplied to a deodorizing stage, in which the undesired flavoring and olfactory or noisome substances are removed. These substances consist mainly of alcohols, ketones, aldehydes, and low-molecular fatty acids and, more recently, of traces of chlorine-containing pesticides.
These substances are basically subjected at elevated temperatures and under reduced pressure in a closed apparatus to distillation in entraining steam. The processes involved have been described in "Chemikerzeitung" 88, pages 412 et seq.
The process can be carried out as a batch process, a semicontinuous process or a continuous process (Fette, Seifen, Anstrichmittel 72, pages 166 et seq., 1970).
The general trend is from the old batchwise process to a continuous process. Whereas the continuous process has the advantage that the continuous feeding and discharge permits heat exchange between the incoming and outgoing products, it has the disadvantage that a remixing may take place in the deodorizer so that the residence time of the product to be deodorized is not exactly defined, as the flow of one part of the oil through the deodorizer is slower than the rest of the oil. A constant defined residence time in the several stages of the continuous process cannot be ensured. Besides, a rapid change of the charge is not possible.
The continuous process has been restricted heretofore to such refined products which can easily be deodorized. With oils and fats which can be refined only with difficulty and in case of a frequent change of product, the semicontinuous process is used more frequently, which is a modification of the discontinuous process. In the semicontinuous process, the charge flows through a closed multiple-stage apparatus, which comprises separate, closed stages which are arranged one over the other. A process control system is used for an optimum control of the temperature, addition of steam, residence time, and the performance of the deodorizing process.
That process permits a variation of the rate, residence time, and change of products within wide limits.
It has been proposed to improve the semicontinuous process by continuously feeding the product to the first stage of the plant and continuously discharging the product from the last stage whereas the intervening stages are intermittently operated in alternation. This practice enables a direct heat exchange in a separate heat exchanger between the hot product as it is discharged and the cold product as it is fed.
The procedure has the disadvantage that owing to the continuous feeding and discharge the treating time of the oil in the first and last stages is not uniform. Here too, portions of the oil are created for a longer or shorter time. A requirement for this process resides in that one and the same product must be processed for long periods of time and there is no frequent change of product.
It is also known to heat the first stage by a closed cycle of a heat carrier and to reheat the cooled heat carrier by the heated product in one of the subsequent stages.
Where a heat carrier consisting, e.g., of heat transfer oil, water or glycerine is used, two stages are provided in most cases with heating and cooling systems and the heat carrier circulates in the interconnected cooling and heating systems. Such plants involve a relatively high capital expenditure. Besides the heat exchange is reduced because there are two heat transfers between the product to be cooled and the heat carrier and between the product to be heated and the heat carrier.